The mayor said he and his wife Amy, who joined him at the podium for the stunning announcement, decided to "write another chapter together" as their three children have left for college.

"[Amy] and I look forward to writing the next chapter in our journey together," an emotional Emanuel said. "I will always be here for the future of this great city. Not as mayor, but in the most important role anyone can play - as a citizen."

His decision comes as the upcoming mayoral race began to take shape, with potential candidates already throwing their hats into the ring, including former Chicago Police Supt. Garry McCarthy, who was fired by Emanuel in the wake of the release of dashcam video showing the fatal police-involved shooting of 17-year-old Laquan McDonald.

The mayor’s announcement took place one day before the start of the trial for Officer Jason Van Dyke, who has been charged with six counts of first-degree murder in the 2014 shooting.

The Emanuel administration was compelled by a court order to release the dashcam video, which showed the white officer shooting the black teen 16 times as he appeared to walk away, more than a year after the shooting and only a few months after the mayor won re-election. His office came under scrutiny and some questioned whether politics played a role in the timing of the release. It also sparked a U.S. Department of Justice investigation into the Chicago Police Department's handling of the case.

Before becoming mayor in 2011, the 58-year-old Emanuel was a Democratic congressman and White House chief of staff to President Barack Obama. He won a historic runoff against Jesus "Chuy" Garcia in his 2015 re-election campaign.

As mayor, Emanuel won some praise for his efforts to upgrade schools, including by adopting a longer school day. He also attracted companies to the city and took measures to ease crisis surrounding the worst-funded pension system of any major U.S. city.

But he was also accused of plowing ahead on policy changes without consulting others. He often reveled in his tough-guy reputation.

And while he's stayed quiet on his future plans, Emanuel - a prolific fundraiser - had been steadily adding to his war chest, with more than $7.5 million in his political committee as of the most recent reporting period ending on June 30.

Numerous political leaders thanked Emanuel for his "service to our city" in wake of the announcement.

"As a mayor, a congressman, and my first White House chief of staff, Rahm Emanuel has been a tireless and brilliant public servant," Obama said in a statement following the news.

"I’ve been blessed to call Rahm my friend," Obama's statement read. "Whatever he chooses to do next, I know he’ll continue to make a positive difference, just as he has throughout his career in public service. And Michelle and I wish Rahm and Amy all the best as they consider this next phase in their lives."